A semiconductor light-emitting device in which a semiconductor element layer including an emission layer is bonded onto a support substrate is known in general. Such semiconductor light-emitting devices are disclosed in Japanese Patent Laying-Open No. 2006-49871 and Japanese Patent Laying-Open No. 2004-235506, for example. These semiconductor light-emitting devices are formed by forming high-quality semiconductor element layers on growth substrates and thereafter bonding the semiconductor element layers onto support substrates different from the growth substrates. The growth substrates can be recycled by removing the growth substrates from the semiconductor element layers after this bonding.
FIG. 18 is a sectional view for illustrating the structure of a conventional light-emitting diode device. The structure of the conventional light-emitting diode device is described with reference to FIG. 16.
In the conventional light-emitting diode device, a GaN-based semiconductor element layer 103 is formed on a support substrate 101 of Si through a bonding layer 102, as shown in FIG. 18. The semiconductor element layer 103 is constituted of a p-type GaN-based semiconductor layer 103a, an active layer 103d and an n-type GaN-based semiconductor layer 103f. 
As a specific structure, the p-type GaN-based semiconductor layer 103a has a thickness of about 200 nm. The active layer 103d having a thickness of about 50 nm and having an MQW (Multiple Quantum Well) structure in which well layers and barrier layers are alternately formed is formed on the p-type GaN-based semiconductor layer 103a. The n-type GaN-based semiconductor layer 103f having a thickness of about 7 μm is formed on the active layer 103d. 
A p-side electrode 105 consisting of a multilayer film of a Pd layer having a thickness of about 3 nm and an Ag layer having a thickness of about 150 nm is formed on the lower surface of the p-type GaN-based semiconductor layer 103a. A barrier layer 106 of Mo having a thickness of about 50 nm is formed on the lower surface of the p-side electrode 105.
An n-side electrode 107 consisting of a multilayer film of a Ti layer having a thickness of about 15 nm and an Al layer having a thickness of about 150 nm from the side of the semiconductor element layer 103 is formed on the upper surface of the semiconductor element layer 103.
An ohmic layer 101a in which a Ti layer having a thickness of about 15 nm and an Al layer having a thickness of about 150 nm are formed in this order is formed on the upper surface of the support substrate 101.
The bonding layer 102 formed between the ohmic layer 101a and the barrier layer 106 is constituted of a first bonding layer 102a of Au having a thickness of about 3 μm formed on the ohmic layer 101a, a second bonding layer 102b of an Au—Sn alloy (Sn content: about 20 mass %) having a thickness of about 3 μm formed on the first bonding layer 102a and a third bonding layer 102c of Au having a thickness of about 100 nm formed on the second bonding layer 102b. 
FIGS. 19 to 21 are sectional views for illustrating a manufacturing process for the conventional light-emitting diode device shown in FIG. 18. The manufacturing process for the conventional light-emitting diode device is now described with reference to FIGS. 18 to 21.
First, a buffer layer 109 of a GaN-based semiconductor having a thickness of about 20 nm is formed on a growth substrate 108 of sapphire by MOCVD (Metal Organic Chemical Vapor deposition), as shown in FIG. 19. Then, the n-type GaN-based semiconductor layer 103f, the active layer 103d and the p-type GaN-based semiconductor layer 103a having the aforementioned thicknesses are formed in this order on the buffer layer 109 by MOCVD. Then, the p-side electrode 105 is formed by forming the Pd layer and the Ag layer having the aforementioned thicknesses respectively in this order on the p-type GaN-based semiconductor layer 103 by electron beam evaporation (EB). Further, the barrier layer 106 of Mo having the thickness of about 50 nm is formed on the p-side electrode 105 by EB.
Then, as shown in FIG. 19, the third bonding layer 102c and the second bonding layer 102b having the aforementioned thicknesses and compositions respectively are formed in this order on the barrier layer 106 by EB.
Then, the ohmic layer 101a is formed by forming the Ti layer and the Al layer having the aforementioned thicknesses respectively in this order on the support substrate 101 by EB, as shown in FIG. 20. Further, the first bonding layer 102a of Au having the thickness of about 3 μm is formed on the ohmic layer 101a by EB.
Then, the growth substrate 108 is arranged on the support substrate 101 so that the first bonding layer 102a and the second bonding layer 102b are in contact with each other, as shown in FIG. 21. Then, the support substrate 101 and the growth substrate 108 are heated/compression-bonded under conditions of about 290° C. and about 200 N/cm2, whereby the first bonding layer 102a and the second bonding layer 102b are bonded to each other. Thereafter a YAG third harmonic laser (wavelength: 355 nm) is applied from the side of the growth substrate 108 toward the buffer layer 109 as shown by arrows in the figure, thereby thermally decomposing parts of the growth substrate 108, the buffer layer 109 and the n-type GaN-based semiconductor layer 103f and removing the growth substrate 108 and the buffer layer 109.
Then, the upper surface of the n-type GaN-based semiconductor layer 103f is polished for removing the buffer layer 109 etc. remaining on the surface, and the n-side electrode 107 is thereafter formed by forming the Ti layer and the Al layer having the aforementioned thicknesses respectively in this order on the n-type GaN-based semiconductor layer 103f, as shown in FIG. 18. Finally, scribing lines are formed on the lower surface (surface to which the semiconductor element layer 103 is not bonded) of the support substrate 101 by dicing, and the support substrate 101 is divided into every semiconductor element layer 103 bonded to the support substrate 101 along these scribing lines. Thus, the conventional light-emitting diode device is formed.
Also in the aforementioned conventional light-emitting diode device, however, bonding strength between the support substrate 101 and the semiconductor element layer 103 is not sufficient. Therefore, there is such a problem that separation may be caused between the support substrate 101 and the bonding layer 102 or between the bonding layer 102 and the semiconductor element layer 103 when the growth substrate 108 is removed, for example. In the conventional light-emitting diode device, further, the semiconductor element layer 103 or the p-side electrode 105 close to the joint surface may be cracked or separated due to a load resulting from heating in bonding. In this case, the operating voltage of the light-emitting diode device may increase or the light-emitting device may emit no light due to non-flowing of an operating current, whereby there is such a problem that reliability of the light-emitting diode device lowers.